1. Field of the Invention
The present invention relates to a cyanate ester based resin-containing insulating composition, an insulating film made therefrom and a multilayer printed circuit board having the insulating film. More specifically, the present invention relates to an insulating composition that contains cyanate ester based resin with high heat resistance, an insulating film prepared therefrom and an insulating film-applied multilayer printed circuit board.
2. Description of the Prior Art
In accordance with a recent trend of miniaturized and digitalized high-frequency electronic apparatuses, electronic parts and boards have been required to have high integration and high density properties. Conventionally, there is known a process for manufacturing a multilayer printed circuit board, in which copper foils are laminated in a press on an internal-layer circuit board having a circuit formed thereon by using as insulating bonding layers several prepreg sheets each prepared by impregnating glass cloth with an epoxy resin, and semi-curing it. This process has, however, had problems including a high cost incurred by large-scale equipment and a long period of time required for molding under heat and pressure by a laminating press, the formation of a fine pattern made difficult by an increased copper thickness due to through-hole plating on an external layer, and high dielectric ratio of the circuit board.
Attention has recently been drawn, as means for solving those problems, to the technique by which organic insulating layers are formed alternately on conductor layers of an internal-layer circuit board for making a multilayer printed circuit board of the build-up type. Therefore, via holes are formed and thus a wiring density is increased according to the build-up process. Additionally, the circuit board having high density and thinner sheet has been made by a laser process. Conventionally, liquid insulation resins and photo-sensitive insulation resins have been used as the insulating layers in the multilayer board of the build-up type. However, because of the formation of an uneven surface on the insulating layers, and the limited selection of the insulating materials for the preparation of the board, it is referred that a semi-solid dry film type insulating materials are used.
In order to improve heat resistance of dry films used as conventional insulating materials, U.S. Pat. No. 6,133,377 refers to an epoxy resin composition suitable for interlayer insulation in a multilayer printed-wiring board of the build-up type, in which heat resistance of insulating films is improved by use of phenolic resin having triazine ring structures and epoxy resin having three or more functional groups with relatively high heat resistance. But, there is a widely recognized need for insulating layers having increased heat resistance by use of a solder devoid of lead.
Accordingly, it is an object of the present invention to alleviate the problems as described above and to provide a cyanate ester-containing insulating composition having an improved high heat resistance.
It is another object of the present invention to provide a dry film type insulating film prepared from the cyanate ester-containing insulating composition, which has an improved heat resistance.
It is still another object of the present invention to provide an insulating film-applied multilayer printed circuit board, having an improved heat resistance.
In an aspect of the present invention, there is provided an insulating composition comprising 1-75% by weight of epoxy resin, 1-60% by weight of cyanate ester resin as represented by the following formulas 1 to 5, up to 20% by weight of a filler, a curing agent and a metal catalyst: 
Wherein, n is integral from 1 to 5. 
Wherein, n is integral from 1 to 5. 
In another aspect of the present invention, there is provided an insulating film prepared from the insulating composition.
In still another aspect of the present invention, there is provided a multilayer printed board having the insulating film.
The present inventors have found that, when epoxy resin, cyanate ester based resin and other additives are used together in suitable proportions, heat resistance can be improved. In the present invention, heat resistance is increased by using cyanate ester based resin having high heat resistance, and forming a net structure through a reaction of hydroxy group in some epoxy resins with cyanate ester. Further, metal compounds are used as a catalyst for the above reaction.
Epoxy resin is a thermosetting resin having excellent electrical and thermal properties, and chemical stability, and is used in a printed circuit board. Epoxy resin includes, but is not limited to, for example, bisphenol type epoxy resins such as bisphenol A and bisphenol F, phenol novolak epoxy resins, and cresol novolak epoxy resins and NBR (Nitrile Butadiene Rubber) modified epoxy resins. In particular, in order to improve fire retardation of insulating layers, brominated bisphenol type epoxy resins and brominated phenoxy epoxy resins may be used.
The epoxy resin is used in an amount of 1-75% by weight on a basis of the total weight of the insulating composition, depending on the amounts of cyanate ester based resin. That is to say, in preparing the insulating composition, the remainder comprises epoxy resin and solvent, based on % by weight. For instance, when the amount of cyanate ester based resin is 60% by weight, epoxy resin is added in an amount of 20-40% by weight depending on the amounts of other components, or in an amount of 30-50% by weight when cyanate ester based resin is used in the amount of 50% by weight.
Cyanate ester based resin having superior electrical and thermal properties forms hard thermosetting net structures by cyclotrimerization in cyanate ester groups during curing, thereby increasing heat resistance and dielectric ratio.
Cyanate ester based resins having the following formulas 1 to 5 can be used alone or in a combination of two or more, and, in particular, PT-15, PT-30, PT-30S, PT-60, PT-60S, CT-90, and BA-230S, which are supplied from Lonza Ltd., Switzerland, may be used. 
Wherein, n is integral from 1 to 5. 
Wherein, n is integral from 1 to 5. 
Cyanate ester based resin is used in the amount of 1-60% by weight, and preferably, in the amount of 25-30% by weight, on the basis of the total weight of the insulating composition. If the amount of cyanate ester based resin exceeds 60% by weight, exothermic reaction rapidly occurs, thus the reaction being difficult to control, and economic loss occurring. On the other hand, if the amount is less than 1% by weight, desired effects cannot be obtained, and also mechanical properties are lowered because of yellowing.
Further, the filler to be removed in the desmear process can be used to increase adhesive strength between an insulating layer and a coating layer. The fillers comprising barium sulfate, barium titanate, silicon oxide powder, amorphous silica, talc, clay, mica powder, can be used in the amount of up to 20% by weight, preferably 10 to 15% by weight.
Examples of amines, amides, imidazoles and anhydrous acids useful as the curing agents for epoxy resins, include, but are not limited to, 2-methylimidazole, 2-phenylimidazole, 2-phenyl-4-phenyl imidazole, bis(2-ethyl-4-methylimidazole), 2-phenyl-4-methyl-5-hydroxymethyl imidazole, 2-phenyl-4,5-dihydroxy methyl imidazole, triazine added imidazole, phthalic anhydride, methyl Nadic anhydride, dicyandiamide, tetrahydrophthalic anhydride, methylbutyltetrahydrophthalic anhydride, hexahydrophthalic anhydride, methylhydrophthalic anhydride, trimellitic anhydride, pyromellitic anhydride and benzophenonetetracarboxylic anhydride. The amounts of the curing agents depend on kinds and amounts of epoxy resin, and can be suitably used by a person skilled in the art. That is, the curing agents are added on the basis of equivalents of epoxy resin.
Meanwhile, metal catalysts serving as reaction catalyts of cyanate ester based resin accelerate the curing, and thus cyanate ester based resin is cured at lower temperature. The metal catalysts are exemplified by manganese naphthenate, zinc naphthenate, cobalt naphthenate, nickel naphthenate, cerium naphthenate, manganese octanoate, zinc octanoate, cobalt octanoate, nickel octanoate and cerium octanoate. The amounts of metal catalysts are dependent on the amount of cyanate ester based resin, and may be appropriately used, considering the reaction of cyanate ester based resin, by a person skilled in the art.
The curable insulating composition may be added with general components used to prepare an insulating film, for example, diluents, binders, coupling agents, solvents, antifoaming agents and rubbers, in the amount commonly used in the art.
When the reactive diluents are used, viscosity can be controlled and thus handling becomes easier. The reactive diluents include, but are not limited to, phenyl glycidyl ether, resorcin diglycidyl ether, ethyleneglycol glycidyl ether, glycerol triglycidyl ether, resol type phenol resin or novolak type phenol resin, and isocyanate compounds.
Binders comprise polyacrylic resin, polyamide resin, polyamideimide resin, polycyanate resin and polyester resin.
Upon preparation of the board, silane coupling agents, such as epoxy silane, or titanium coupling agents is used to increase adhesive strength between a conductive layer and an insulating layer.
Examples of the solvents include acetone, methyl ethyl ketone, cyclohexanone, ethyl acetate, butyl acetate, cellosolve acetate, propylene glycol monomethyl ether acetate, ethylene glycol monobutyl ether acetate, cellosolve, butyl cellosolve, carbitol, butyl carbitol, xylene, dimethylformamide and dimethylacetamide, with consideration of solubility and compatibility of the resins and other additives used in the present invention.
In the desmear process for increasing adhesive strength between the insulating layer and the coating layer upon the preparation of the board, the added rubber is swelled and thus allows the filler to be easily discharged, thereby increasing the adhesive strength and improving the elongation ratio. The rubbers comprising polybutadiene rubber, modified polybutadiene rubber, acrylonitrile-butadiene rubber, acrylonitrile-modified polybutadiene rubber and NBR modified epoxy resins can be used alone or in mixture. In addition, the rubbers are used in the amount of 1-20% by weight, based on the total weight of the insulating composition. The rubber amount exceeding 20% by weight results in decrease of the adhesive strength.
The curable heat resistant composition can be coated as solution phase or applied as the insulating layer in a dry film form of semi-solid phase, upon the preparation of the multilayer printed circuit board of the build-up type.
The cyanate ester based resin-containing insulating composition of the present invention can be prepared in the semi-solid dry film form, according to general methods known in the art. In this regard, the composition is formed in the film by use of a roll coater or a curtain coater, and then applied on the board, thus being used as the insulating layer upon the preparation of the multilayer printed board of the build-up type.
For instance, the dry film prepared from the insulating composition of the present invention is laminated on CCL (copper clad laminate) used as an internal layer on the preparation of the multilayer printed board.
A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.